Power-level indicator



March 14, 1933.

.1. w. HORTON 1,901,344 POWER LEVEL INDICATOR Filed June 27, 1931 ,62 s u I4 4 .o'

O 0 3O 32 O r 6 as 40 v 26 0' f 52 4-9 24 54 I 47 56 72 as 64 [rave-@20 Josephfilboyton,

Patented Mar. 14, 1933 UNITED STATES PATENT oFFicn JOSEPH WARREN HORTON, OF CAMBRIDGE, MASSACHUSETTS, ASSIGNOR-'TO GENERAL RADIO COMPANY, OFCAMIBRIDGE, MASSACHUSETTS, A CORPORATION OF M- Application filed June 27,

The present invention relates to electric systems and methods, and more particularlyto power-level indicators for indicating the, level of power in a system, such as a telephone transmission circuit.

In many types of electric systems, where power is transmitted from a source of electric power to a load, it is desirable-to measure or control the electricpower transmitted along the junction between the source and the load. The importance of maintaining a careful check upon the signal amplitude at various points in voice-transmission circuits, for example, is well recognized by acoustic engineers. Too low alevel means interference from means overloading of amplifiers and reproducers and the introduction of crosstalk into I nearby channels.

An object of the invention is to provide a new and improved method of and apparatus for efiecting measurements of the above 'described character, to the ends that the efli-- ciency of the measurements may be enhanced, the measurements themselves more easily made, and the apparatus employed sim iiified and cheapened. Other and further 0 jects will be explained hereinafter.

The invention will now be explained in connection with the accompanying drawing, the

single figure of which is a diagrammatic view 7 of circuits and apparatus arranged and constructed according to a preferred embodiment of the invention.

, the junction.

In the accompanying drawing, electric power is assumed to be delivered from a source or generator 1 to a load 3 along a" junction, illustrated as comprising conductors 5, 7. 7 One side of the junction 5, 7 may, for example, be an incoming telephone line,

and the other side the outgoing line. It is desired to measure the electric power de-" livered across this junction 5, 7 to the load 3. If the impedance of the load 3 is known, the power delivered may be completely determined by a measurement of. the voltage at The junction the incoming and outgoing lines by conductors 2, 4 that constitute an inputconnection background noises; too high a level is tapped at a point between rowan-Lever. mmca'ron 19 1; Serial no. 547,444. for a.- suitable attenuation network. This netrange of the instrument and is so designed as to'maintain the input impedance constant rebe so calibrated as to permit reading directlythe electrical power delivered to the load 3.

Thiscalibration is based on the known value.

of the load impedance. Both the attenuation network and the indicating meter may be calibrated in logarithmic"- units, such as the decibel, by which the power actually delivered ,work' serves as a multiplier for increasing the to the load is expressed as the logarithm of its ratio to some arbitrary reference power. The actual powerlevel at which energy is de-' livered to the load may, under these conditions,ybe obtained by taking the sum ofthe readings of the multiplier switch and ofthe meter.

The network may assume any of a number of different forms, but is illustrated, for simplicity, as a single, inert, L-type, attenuation section consisting of a variable series resistor branch or arm 10 and a variable shunt resistor branch or arm 12. The meter 14 is connected in the output connection, and is provided with an indicator 16 adapted'to travel over a suitably graduated-scale 18.

' The meter 14 may be of any desired type, such as the thermionic, thermocouple, and the hotwire types, but it is preferred to employ a voltmeter of the copper-oxide-rectifier type.

The indicator 16 is'slightly damped to prevent any undue overthrow'on sudden, shorttime-interva'l surges 'or sudden variations,

such as take place at-"normal syllabic frebut the construction is suchas to perquencies,

mit the needle to respond'to longer impulses,-

or those varying at the desired rate. The de-.

sign may be such that the graduation of the scale 18 shall be approximately proportional. I

to the mean syllabic frequency, or any other desired value, and may cover the whole voice spectrum. The indicator doesnot follow each sudden variation, but reads a mean power value; This result may be brought about in any desired way, as by suitably designin the structure and the shape of the shell .9. ut

which is wound the coil for controlling the movementsof the indicator.

The meter scale and the attenuation network are preferably calibrated in lo arithmic, electrical transmission units, pre erably decibels, for any desired load impedance, preferably 500 ohms. This method of measuring power may be defined as follows. Let P represent the'power delivered across junction 5, 7 fromthe source 1 to the load 3, and

let P represent an arbitrarily chosen refer ence power as, for example, 0.006 watt. The

j I -The number of decibels tion is to be added to the power actually delivered to the load may then be expressed in terms of the arbitrary reference level by means of the equation where N is the number of decibels expressing the ratio ofthe power actually transmitted to the reference power. Using the quantities already mentioned as representing typical cases, namely, a load impedance of 500 ohms and a reference power level of 0.006 watt, the reference power is across the junction is Whenever the power-level-indicating instrument is connected across the input junction to a 500-ohm load, the power level is N 20 logdecibelsabove reference levels, where V is the actual voltageexisting at the junction.

Should the power-level-indicating instru to occupy any desired number of positions,

sixteen being illustrated. The contact arm 24 contacts with a pair of contact members 26, 34, in its first position; a second pair of contact members 28, 36, in its second position a third pair of contact members 30, 38 in its third position; afourth pair of contact members 32-, 40 in its fourth position, and so on. Any desired number of pairs of contact members may be employed (sixteen being illustrated), depending upon the range for which the instrument is designed.

obtained when the voltage the contact members 36, 38, 40

ductors 64, 68, 72

pedance is the impedance To explain the operation, let it be assumed that the indications of the indicator 16 on the scale 18 are correctly given on the scale when the contact arm 24 occupies its first position, in which it contacts with the contact members 26 and 34. Then, when the contact arm 24 occupiesits second position, in which it contacts with the contact members 28 and 36, the readings of the indicator 16 on the scale 18 should be added to the reading corresponding to the position of the contact arm- 24, when occupying this second position; and similarly, when the contact arm occupies its third and fourth and other positions, in which it contacts with the pair of contact members 30 and 38 and the pair of contactmembers 32 and 40, respectively; and so on. If the range is 16 decibels in the initial position of the contact arm 24, the range may thus be increased by designing the instrument so that the scale may be graduated'in steps of 2 decibels to cover the range, for example, from -10 decibels to +6 decibels, so as to read directly when across a 500-ohm line. The net-work will then be designed so that the adjustments of the contact arm 24 shall correspond to steps of 2 decibels.

With the instrument so designed, and because the its are logarithmic, the total power level, in decibels, is obtained by adding the scale reading to the multiplier readings. The instrument will thus represent the level as so many decibels above or below the reference level.

A feature of the invention resides in having the input impedance substantially the same irrespective of the adjustment of the contact arm 24. To this end, the branches 10 and 12 are respectively divided into suit ably designed sections 41, 43, 45.. and 47,49, 51, respectively between successive terminals 42, 44, 46, 48 and 56, 54,-.52, The terminals 42, 44, 46, 48 are respectively connected with the contact members 26, 28, 30, 32 by conductors 58, 62, 66, 70 The terminals 56, 54, 52 are similarly connected with by conrespectively. The contact member 34 is open-circuited.

When the contact arm 24 occupies its first, or illustrated position, in which it contacts with the contact members 26 and 34, a cir-j cuit is established from the conductor 7, by way of conductors 4vand 8, to the meter 14. The shunt resistor 12 is at this .time opencircuited and the series resistor 10 is efi'ectively reduced to zero. I The circuit continues,

by way of the conductor 6, through the pivot '25, the arm 24 and the contact member 26,

and by way of the conductors 58 and 2, back to the conductor 5. In this position of the contact arm, therefore, the meter 14 is thrown directly across the line, and the input imof the meter.

When the contact arm 24 occupies its sec-,

ond osition, in which it bridges the contact mem rs 28 and 36, the circuit is as follows: from the conductor 7, by way of the conductor 4, to the conductor 8, and the'resistor 12 in parallel. The current in the conductor 8 passes through the meter 14 and the point to the contact arm 24, as before described. The circuit continues by way of the contact members 36 and 28, and by way of the conductor 62, to the terminal 44, so that the circuit includes the portion 41 of the resistor 10 between the terminals 42 and 44.

' The parallel branch circuit'through the resistor 12 contains the'whole resistor 12, the circuit continuing from the point 56, by way of the conductor 64, to the terminal 36.

In its third position,thecontact arm 24 bridges the terminals and 38, with the result thatthe series circuit contains the portions 41 and 43 of the resistor 10 between the terminals 42 and 46, and the shunt circuit contains the whole of the resistor 12, except the portion 47 between the terminals 54 and 56. In the fourth position of the contact arm 24, the portions 41', 43 and 45 of the series resistor 10 and the portions 47 and 49 only of the resistor 12, between the terminals 56 and 52, including the meter, are connected in circuit; and "so on.

The resistors comprising the arms 10 and I 12 are soconstructed asto present a purely sibility resistive load, independent of frequency, throughout the range over which the instrument'is to be used, thus eliminating all posof changing the-frequency characteristics of the junction.

The instrument, of course, will be so designed as to have a high input impedance in order that it shall draw but little power from the'junction 5, 7, so as not to react on the junction, and so that the system shall operate as in telephone circuits, and to control voiceas though the instrument were not connected with the junction.

Among the uses to which the present invention may be put. are monitoring, as well asmeasuring the voice-power level, as in telephone conversations, power and voltagemeasurements, amplifier gain and circuitloss'. determinations, and equalization and measurements on voice circuits.- It may be used to measure transmission characteristics,

signal amplification in 'radio broadcasting, and the phonographic and film recording of speech. The inventionis adapted for use in all kinds of transmission and recordingcircuits and is useful wherever it is desired to control,- check'or indicate the electrical power transmitted from an electric source to a load. It may be used to measure the performtesting. I e

When connected across a transmlsslon line carrying a program of .voice and music, the

time of the circuit, both while in use and for instrument will indicate to an operator be- I tween wha't approximate power limits or levels the sound is being deliveredpast its terminals. "Again, it having been'found, by

experience, at just what power level 1nter-. fering speech becomes bothersome on a telehone circuit, and as this invention indicates ust what the speech power level is, theoperator will be enabled to hold the speech hetow lthe interfering level and above the noise eve Modifications may obviously be made by persons skilled in the art, and all such are considered to fall within the spirit and scope of the invention, as' defined in the appended claims.

What is claimedis:

1. A method of measurin the alternating electric power transmitted om a source of alternating electric power to a load along a. junction, the said method comprising connecting across the junction a network having a highimpedance in order that itshall draw but'littl'e powerfrom the junction, the network being provided with an indicator having a predetermined of movement, adj usting the network to vary the significance of the indications of the indicator, and maintaining the said impedance substantially constant irrespective of the adjustment of the network, the network being of such nature as to present a purely resistive load, independent e of frequency, throughout the said range.

2. In a power-level'indicator, a source of alternating electric power, a load,'a junction connecting the source with the load, a net work connected with the junction and having a high impedance in order that it shall draw but little power from the junction, an indi-' cator controlled by the network; having a predetermined rangeof movement, andmeans for adjusting the network to vary the significance of the indications of the indicator and for maintaining the said impedance substantially constant irrespective of the adjustment of the network, the networkv being of such nature as to present a purely resistive .load, independent of frequency, throughout the said range. I

3. Ina power-level indicator, a source of alternating electric power, a load, a junction connecting the source with the load, a net-' theadjustment of'the network and the network being of such nature 'as to present a purely resistive load, independent. of frequency, throughout the said range.

'4. In a power-level indicator, a source of alternating electric power, a load, a junction "connecting the sourcefwiththeload, a network having an input connection and an output connection, the input connection being connected with the junction, thev input having a high impedance in order that the network shall draw but little power from the junction, a meter connected with the output connection and provided-with an indicator having a predetermined range of movement, means for adjusting-the network to vary the significance of the indications of the indicator, and means for maintaining the impedance of the network subflantially constant irrespective of the adjustment of the -the input having a high impedance in order that the network shall draw but little power from the junction, a meter connected with the output connection and provided with am indicator-having a predetermined range of movement, and means for adjusting! the series and shunt arms to ary the significance of the indications of the indicator and for maintaining the impedance of the stant irrespective of the adjustment of the network, the network being of 'such nature as to present a purely resistive load, independent of frequency, throughout the said range.

7. In a power-level indicator, a source of alternating electric power, a load, a junction connecting the source with the load, a network having a series arm and a shunt arm, an .input connectlon and an output connection connected with the network, the input connection being connected with the junction, the input having a high impedance in order that the network shall draw but little power from the junction,

a meter connected with the outputconnection and provided with an indicator having a predetermined range off movement, and means for adjusting the series arm and the shunt arm to vary the,significance of the indications of the indicator and for maintaining the impedance of the network and the meter substantially constant irrespective of the adjustment of'the series arm and the shuntarm, the network being of'such nature as. to present a purely resistive load, independent of frequency,- throughout the said range.

In testimony'whereof, I have hereunto subscribed m name.

J SEPH WARREN HORTO N.

network and the meter substantially constant irrespective of the adjustment of the series and shunt arms, the network being of such nature as to present a purely resistive load,

independent of frequency, throughout the said range.

6. In a power-level indicator, a source of alternating electric power, a load, a junction connecting the source with the load, a network having an input connection and an output connection, the network being calibrated in lo arithmic power units, the input connection being connected with the junction, the input having a high impedance in order that the network shall draw but little power from the junction, a meter connected with the output connection and provided with a scale calibrated in the said logarithmic power units and with an indicator having a predetermined range of movement over the scale, means for adjusting the network to vary the significance of the indications of the indicator, and means for maintaining. the impedance of the network substantially con- 

